The technical field of this invention is methods for screening for transdominant effector peptides and RNA molecules selected inside living cells from randomized pools.
Signaling pathways in cells often begin with an effector stimulus that leads to a phenotypically describable change in cellular physiology. Despite the key role intracellular signaling pathways play in disease pathogenesis, in most cases, little is understood about a signaling pathway other than the initial stimulus and the ultimate cellular response.
Historically, signal transduction has been analyzed by biochemistry or genetics. The biochemical approach dissects a pathway in a xe2x80x9cstepping-stonexe2x80x9d fashion: find a molecule that acts at, or is involved in, one end of the pathway, isolate assayable quantities and then try to determine the next molecule in the pathway, either upstream or downstream of the isolated one. The genetic approach is classically a xe2x80x9cshot in the darkxe2x80x9d: induce or derive mutants in a signaling pathway and map the locus by genetic crosses or complement the mutation with a cDNA library. Limitations of biochemical approaches include a reliance on a significant amount of pre-existing knowledge about the constituents under study and the need to carry such studies out in vitro, post-mortem. Limitations of purely genetic approaches include the need to first derive and then characterize the pathway before proceeding with identifying and cloning the gene.
Screening molecular libraries of chemical compounds for drugs that regulate signal systems has led to important discoveries of great clinical significance. Cyclosporin A (CsA) and FK506, for examples, were selected in standard pharmaceutical screens for inhibition of T-cell activation. It is noteworthy that while these two drugs bind completely different cellular proteinsxe2x80x94cyclophilin and FK5O6 binding protein (FKBP), respectively, the effect of either drug is virtually the samexe2x80x94profound and specific suppression of T-cell activation, phenotypically observable in T cells as inhibition of mRNA production dependent on transcription factors such as NF-AT and NF-xcexaB. Libraries of small peptides have also been successfully screened in vitro in assays for bioactivity. The literature is replete with examples of small peptides capable of modulating a wide variety of signaling pathways. For example, a peptide derived from the HIV-1 envelope protein has been shown to block the action of cellular calmodulin.
A major limitation of conventional in vitro screens is delivery. While only minute amounts of an agent may be necessary to modulate a particular cellular response, delivering such an amount to the requisite subcellular location necessitates exposing the target cell or system to relatively massive concentrations of the agent. The effect of such concentrations may well mask or preclude the targeted response.
Thus, it is an object of the present invention to provide methods and compositions for the effective introduction of random libraries into cells to screen for bioactive compounds.
Relevant Literature
Mann et al. (1983) Cell 33, 153-159, Pear et al. (1993) Proc. Natl. Acad. Sci. USA 90(18):8392-6 and WO 94/19478 describe the BOSC and BING retroviral systems useful as delivery vectors for the disclosed methods.
Scott and Craig (1994) Current Opinion in Biotechnology 5:40-48 review random peptide libraries. Hupp et al. (1995) describe small peptides which activate the latent sequence-specific DNA binding function of p53. Palzkill et al. (1994) report the selection of functional signal cleavage sites from a library of random sequences introduced into TEM-1-lactamase.
The invention provides methods and compositions for screening for transdominant bioactive agents such as pharmaceuticals. The invention accesses molecules or targets within living cells and provides for the direct selection of those bioactive agents with desired phenotypic effects.
In one aspect of the invention, methods for screening for a transdominant bioactive agent capable of altering the phenotype of a cell are provided. The methods comprise the steps of a) introducing a molecular library of randomized candidate nucleic acids into a plurality of cells, wherein each of said nucleic acids comprises a different nucleotide sequence; b) screening the plurality of cells for a cell exhibiting an altered phenotype, wherein the altered phenotype is due to the presence of a transdominant bioactive agent. The methods may also include the steps of c) isolating the cell(s) exhibiting an altered phenotype, d) isolating a candidate nucleic acid from the cell(s).
The invention further provides methods for isolating a target molecule using either a candidate nucleic acid or the expression product of a candidate nucleic acid.
In an additional aspect, the candidate nucleic acids of the invention are linked to fusion partners.
In a further aspect, the invention provides methods for screening for a transdominant bioactive agent capable of altering the phenotype of a cell. The methods comprises the steps of a) introducing a molecular library of randomized candidate nucleic acids into a first plurality of cells, wherein each of the nucleic acids comprises a different nucleotide sequence; b) contacting the first plurality of cells with a second plurality of cells; and c) screening the second plurality of cells for a cell exhibiting an altered phenotype.
In an additional aspect, the present invention provides molecular libraries of retroviruses comprising different randomized nucleic acids, and cellular libraries containing the retroviral libraries.